Car telephone systems whose operation and control are integrated in a central unit for also controlling a car radio, a navigation system, an onboard computer etc. are installed, in particular, in vehicles of higher quality. This central operating and control unit is regularly connected to the corresponding devices via an electronic control and communication bus of the vehicle. This control bus is also used to transmit vehicle-relevant and safety-relevant data, for example signals which reflect actuation of the accelerator pedal or brake pedal.
It is known practice to provide the telephone system with its own telephone which is connected to a vehicle antenna and to a microphone which is installed in the vehicle and to loudspeakers in order, in this manner, to make it possible to make a telephone call in the hands-free mode, i.e. without the need to hold the telephone. A system of this type is very complicated and regularly requires the car telephone to be registered as a further mobile telephone in addition to a portable mobile telephone which the car user generally already has with him. Some mobile radio providers make it possible for the installed car telephone to be used with a second card which belongs to the contract for the portable mobile telephone, so that the calls which are made using the car telephone can be invoiced using the contract which has been concluded for the portable mobile telephone.
It is known practice, in particular for car telephone systems which can be retrofitted, to install a telephone control box in the vehicle and to connect the latter to a holder into which a portable mobile telephone can be inserted. In this case, the holder may be designed in such a manner that it connects the mobile telephone to a vehicle antenna, so that the antenna on the mobile telephone can remain switched off when making a telephone call. The call is then handled using the mobile telephone which has been inserted into the holder. Since this uses a relatively large amount of power from the rechargeable battery of the mobile telephone, the holders are regularly provided with a charging device for the mobile telephone.
Use is increasingly being made of mobile telephones which have a transmitting and receiving device for data and audio signals, transmission generally being effected in accordance with a wireless communication standard, the BLUETOOTH® standard (BLUETOOTH® is a registered trademark of Bluetooth SIG, Inc.). If the control box which has been installed in the vehicle and the mobile telephone each have a transmitting and receiving device of this type, the function of the mobile telephone can be controlled by the control box or by the central operating device of the vehicle by virtue of the control signals being transmitted to the mobile telephone via the wireless connection. Conversely, audio signals are wirelessly transmitted from the mobile telephone and allow the telephone call to be made via the transmitting and receiving device of the mobile telephone but using the appropriate devices in the vehicle, for example a hands-free device or a headphone/microphone set. The audio signals are transmitted using the BLUETOOTH® wireless communication standard, thus with a HFP (Hands Free Profile) signal or a HSP (Head Set Profile) signal. This solution is convenient since there is no need to provide a holder for the mobile telephone and corresponding installation in the vehicle can be omitted. However, the disadvantage is that the transmitting and receiving device of the mobile telephone is used with the antenna of the mobile telephone, with the result that a certain intensity of the transmitted radiation occurs in the vehicle interior. It is also disadvantageous that the current loading on the mobile telephone is high in this mode, with the result that more attention must be paid to the state of charge of the mobile telephone which often does not occur in practice.